Ni-Base Single Crystal Superalloy with Enhanced Creep Property

ABSTRACT

The present invention provides Ni-base single crystal superalloy with good high-temperature property, particularly long creep life and excellent resistance to creep deformation, by adjusting content of Al and Ti that form a gamma prime (γ′), a major hardening phase of the Ni-base single crystal superalloy. The Ni-base single crystal superalloy comprise Co: 11.5˜13.5%, Cr: 3.0˜5.0%, Mo: 0.7˜2.0%, W: 8.5˜10.5%, Al: 3.5˜5.5%, Ti: 2.5˜4.5%, Ta: 6.0˜8.0%, Re: 2.0˜4.0%, Ru: 0.1˜2.0% in Weight %, and the rest is Ni and other inevitable impurities. And composition ratio of Al/Ti is 0.7˜2.2. In addition, the superalloy has a mixed structure of the γ matrix and γ′ particles.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims foreign priority under 35 U.S.C. §119(a)-(d) toApplication No. KR 10-2011-0023290 filed on Mar. 16, 2011, entitled“Ni-Base Single Crystal Superalloy with Enhanced Creep Property,” theentire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to Ni-base single crystal superalloy,particularly, Ni-base single crystal superalloy with enhanced creepresistance and creep rupture time at high temperature by adjustingcontent of elements that form gamma prime (γ′), a hardening phase.

BACKGROUND

Ni-base superalloys are widely used as materials for major parts likeblades and vanes of gas turbines for aircraft engines and for powergeneration. The application of single crystal superalloy increasedbecause of its excellent high temperature mechanical properties comparedwith conventionally cast polycrystalline superalloy and directionallysolidified superalloy.

Single crystal superalloy is strengthened by the precipitates ofintermetallic γ′(L1₂ structure), a hardening phase having orderedstructure within a matrix, and its matrix reinforced by adding alloyingelements like W, Mo, Re, etc.

However, as environmental issues like global warming are on the rise,the necessity to enhance efficiency of the gas turbines by increasingthe operation temperature becomes a matter of big concern. Therefore,temperature capability and creep life of blades and vanes used in themost extreme environment among gas turbine parts are getting important.Accordingly, development of single crystal superalloy with better creepproperty at high temperature than prior art is becoming more important.

The generation of single crystal superalloy is classified by Re content,an alloying element; that is, the 1^(st) generation contains no Recontent, the 2^(nd) generation contains 3% of Re, the 3^(rd) generationcontains 6% of Re, etc. Also, the 4^(th) generation with Ru addition wasrecently developed. Although temperature capability and creep resistanceat high temperature have been improved as the generation is updated, theprice of superalloy also went up because of an increase in addition ofexpensive elements such as Re, Ru, etc. For this reason, CMSX-4 (U.S.Pat. No. 4,643,782), the 2^(nd) generation single crystal alloycontaining 3% of Re developed by Cannon Muskegon, U.S., is being mostcommonly used at the present time.

In order to satisfy the need of developing single crystal superalloywith excellent temperature capability and creep resistance, adjustingcontent of other alloying elements while minimizing expensive alloyingelements is regarded as an effective alloying design method. In case ofparts that are used at high temperature, creep resistance is also a veryimportant factor to be considered for alloying design because deformedparts cannot be used properly as per their original purposes or lowerefficiency although creep lifetime is important.

As mentioned above, solid solution hardening elements such as W, Mo, Re,etc. can be adjusted in order to improve creep property of superalloy.In addition to this, creep property can be also enhanced by adjusting Alor Ti content that forms γ′, a hardening phase having ordered structure(L1₂ structure). It is necessary to study the latter because it cansuppress price raise compared with the former that enhances creepproperty through solid solution hardening by adding expensive elementssuch as Re, etc.

SUMMARY OF THE INVENTION

Accordingly, the present invention aims to provide Ni-base singlecrystal superalloy with good high-temperature property, particularlylong creep life and excellent resistance to creep deformation, byadjusting content of Al and Ti that form a gamma prime (γ′), a majorhardening phase of the Ni-base single crystal superalloy.

Ni-base single crystal superalloy with good creep property in thepresent invention consists of Co: 11.5˜13.5%, Cr: 3.0˜5.0%, Mo:0.7˜2.0%, W: 8.5˜10.5%, Al: 3.5˜5.5%, Ti: 2.5˜4.5%, Ta: 6.0˜8.0%, Re:2.0˜4.0%, Ru: 0.1˜2.0% in Weight %, and the rest is Ni and otherunavoidable impurities. At this time, composition ratio of Al/Ti is0.7˜2.2. The above superalloy may have a mixed structure of the γ matrixand γ′ particles.

According to the Ni-base single crystal superalloy with good creepproperty of the present invention, it is possible to obtain alloy withprolonged creep rupture life and significantly improved Time to 1% CreepStrain representing resistance to creep deformation through increasingmisfit, a difference of lattice constant between the γ matrix and γ′particles, and reducing stacking fault energy, by producing singlecrystal superalloy consisting of Co: 11.5˜13.5%, Cr: 3.0˜5.0%, Mo:0.7˜2.0%, W: 8.5˜10.5%, Al: 3.5˜5.5%, Ti: 2.5˜3.5%, Ta: 6.0˜8.0%, Re:2.0˜4.0%, Ru: 0.1˜2.0% in Weight %, and the rest containing Ni and otherunavoidable impurities.

The foregoing and other objects, features, aspects and advantages of thepresent invention will be more clearly understood from the followingdetailed description with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph that shows creep life and variation of creep strainwith time when creep tests are performed with Ni-base superalloyaccording to the present invention at the condition of 950° C./355 MPa.

FIG. 2 is a photo of microstructure observed with a TEM(TransmissionElectron Microscope) after a creep experiment for Test Material 1,Comparative Test Materials 1 and 2 of the present invention.

DETAILED DESCRIPTION

Ni-base single crystal superalloy with good creep property will beexplained in the following embodiment. The creep property here meansresistance to creep deformation as well as creep rupture life that isessential to use superalloy at high temperature. The said Ni-basesuperalloy has the following major features.

Ni-base single crystal superalloy with good creep property in thepresent invention obtains high temperature strength by bothprecipitation hardening and solid solution hardening.

A hardening phase, γ′ having ordered structure(L1₂ structure) forms byadding Al and Ti in the γ-phase matrix, and the matrix is reinforced byadding solid solution hardening elements like W, Mo, Re, Ru, etc.Particularly, the Ni-base single crystal superalloy in the presentinvention is characterized by maximizing creep property by changingstacking fault energy through increasing Ti content and decreasing Alcontent, and also characterized by more enhanced creep properties thancommonly used alloy.

In order to get the Ni-base single crystal superalloy with good creepproperty in the present invention, master ingots are cast using vacuuminduction melting process. Then, single crystal specimens are producedfrom each master ingot respectively by the Bridgman method. And then,microstructure consisting of two phases of γ and γ′ can be obtained byapplying heat treatment to the specimens.

[Composition of the Alloy]

The Ni-base superalloy in the present invention has the followingcomposition for each element. The reason for limiting amounts of eachelement will be explained here. The below weight % is gained byconverting the amount added to weigh while defining the entire Ni-basealloy as 100. In order to make it easy, explanation of Ni and otherinevitable impurities will be omitted.

(1) Cobalt (Co) : 11.5˜13.5%

Cobalt influences solution treatment temperatures by changing a γ′solidus, a major hardening phase of Ni-base superalloy, and γ solidus, amatrix, in addition to solid solution hardening. It also improves hightemperature corrosion resistance. Creep property becomes worse if Cocontent is less than 11.5%, while it is difficult to decide heattreatment conditions because the temperature range of solution treatmentbecomes narrow if Co content is more than 13.5%.

(2) Chrome (Cr) : 3.5˜5.0%

Chrome improves corrosion resistance of superalloy, however, the amountof Chrome is limited because it may produce carbides or TCP(Topologically Close Packed) phases which are detrimental to creepbehavior. Corrosion resistance becomes bad if Cr content is less than3.5%, while more than 5.0% Cr content may lower creep property andcreate TCP phases that negatively influence mechanical properties incase of long exposure at high temperature.

(3) Molybdenum (Mo) : 0.7˜2.0%

Molybdenum improves property of superalloy at high temperature as asolid solution hardening element. However, a large amount may increasedensity and create TCP phases. It is hard to expect solid solutionhardening effect under 0.7%, while more than 2.0% increase the density.

(4) Tungsten (W) : 8.5˜10.5%

Tungsten is an element that enhances creep strength by solid solutionhardening. However, a large amount may increase density, and lowertoughness, corrosion resistance and phase stability. In addition, apossibility of casting defects like freckles increases at a time ofsingle crystal and directional solidification. Accordingly, more than8.5% Tungsten is added for improving high temperature strength whileTungsten content is limited to 10.5% in order to inhibit undesirableeffects.

(5) Aluminum (Al) : 3.5˜5.5%

Aluminum is an essential element to improve high temperature creepproperty because it is a constitutive element of γ′, a major hardeningphase of Ni-base superalloy. In addition, it improves oxidationresistance. However, creep strength lowers under 3.5% while mechanicalproperty may become worse due to precipitate of excessive γ′ phases incase of adding more than 5.5%. Although absolute quantity of Al isimportant, an association with Ti content, another γ′ phase formingelement, is also important.

(6) Titanium (Ti) : 2.5˜4.5%

Titanium, like Aluminum, improves creep strength as a constitutiveelement of a γ′ phase. Particularly, more than 2.5% should be added inorder to enhance creep property because addition of Ti increases misfitand decreases stacking fault energy. However, the amount should belimited to 4.5% because excessive addition may reduce oxidationresistance and lower phase stability.

(7) Tantalum (Ta) : 6.0˜8.0%

Tantalum improves creep strength by hardening ion resis. In addition,partitioning of tantalum to interdendritic region increases the densityof interdendritic liquid, resulting in inhibition of freckles, one ofcasting defects. Therefore, more than 6.0% content is required. But ifmore than 8.0% are added, harmful δ phases can be precipitated.

(8) Rhenium (Re) : 2.0˜4.0%

Rhenium, a solid solution hardening element, greatly contributes toimprovement of creep property because its diffusivity is very low. Inother words, Rhenium considerably improves resistance to creepdeformation as well as creep life of superalloy. Yet, a large quantitylowers phase stability, increases density and raises the price,therefore, the present invention limited the amount of Rhenium to2.0˜4.0%.

(9) Ruthenium (Ru) : 0.1˜2.0%

Ruthenium improves high temperature property by inhibiting creation ofTCP phases through broadening the solid solution range of γ′ phase andcontributing to homogenization of segregation. Accordingly, in thepresent study Ruthenium is added to enhance resistance to creepdeformation as well as creep life of superalloy. However, the amount islimited to 0.1˜2.0% because the price of superalloy becomes expensiveand the density increases if a large quantity of Ruthenium is contained.

The present inventions will be explained in more detail through thefollowing embodiments.

[Table 1] shows the chemical composition of single crystal superalloyaccording to the present invention and alloy compared with the saidsuperalloy.

According to [Table 1], Test Material 1 presents the composition ofNi-base alloy with 4.5 weight % of Al and 3.0 weight % of Ti added,while Test Material 2 shows a case with 5.0 weight % of Al and 2.5weight % of Ti added. On the contrary, Comparative Test Material 1 isalloy with 5.5 weight % of Al and 1.0 weight % of Ti added, andComparative Test Material 2 is CMSX-4 that is being most commonly usedat the present time.

TABLE 1 Alloy Co Cr Mo W Al Ti Ta Re Ru Hf Al/Ti Test 1 11.59 3.99 0.988.54 4.45 3.00 6.92 2.98 0.98 0 1.48 Materials 2 11.57 4.07 1.02 8.595.02 2.51 7.01 2.97 0.97 0 2.00 Comparative 1 11.66 4.07 1.03 8.68 5.471.02 6.95 3.02 1.02 0 5.36 Test Materials 2 9.60 6.40 0.61 6.40 5.651.01 6.50 2.90 0 0.10 5.45

The above Test Materials and Comparative Test Materials were produced asfollows. First of all, master ingots were cast using vacuum inductionmelting process. Then, single crystal specimens of 15 mm diameter and180 mm length were produced by the Bridgman method with withdrawal rateof 4.0 mm/min. And then, microstructure consisting of two phases of γand γ′ can be obtained by applying heat to the specimens.

[Table 2] shows creep life and time to 1% creep strain when creep testsare conducted by applying stress of 355 MPa at 950° C. with the abovealloys. [FIG. 1] is a graph that shows variation of creep strain withtime when creep tests are performed at the condition of 950° C./355 MPa.

TABLE 2 Comparative Comparative Test Test Test Test ClassificationMaterial 1 Material 2 Material 1 Material 2 Creep Rupture 301.8 270.2211.7 123.1 Time (Hour) Time to 1% Creep 197.0 151.9 112.0 57.0 Strain(Hour)

As we know from [Table 2] and [FIG. 1], creep property of Ni-base alloyis greatly dependent on content of Al and Ti, gamma prime (γ′) formingelements. That is, it is found that Test Material 1 with relativelyhigher Ti content and lower Al content shows significantly longer CreepRupture Time and Time to 1% Creep Strain than other Test Materials orComparative Test Materials. Of course, optimizing contents of otheralloying elements is necessary in order to improve creep property byadjusting the gamma prime phase forming elements.

In the concrete, Creep Rupture Time of Test Materials 1˜2 withrelatively higher Ti content and lower Al content was 270.2˜301.8 hourswhile Time to 1% Creep Strain was 151.9˜197.0 hours. On the other hand,Comparative Test Materials 1˜2 presented 123.1˜211.7 hours of CreepRupture Time and 57.0˜112.0 hours of Time to 1% Creep Strain. Therefore,it was found that Test Material 1˜2 of the present invention showedlonger Creep Rupture Time and Time to 1% Creep Strain compared withComparative Test Material 1˜2.

FIG. 2 is a photo of microstructure observed with TEM(TransmissionElectron Microscope) after a creep experiments for Test Material 1,Comparative Test Materials 1 and 2 of the present invention.

According to FIG. 2, although superdislocation is observed mainly insideof γ′, a hardening phase, after the experiment in case of ComparativeTest Materials 1 and 2, stacking fault is observed in Test Material 1.This is because formation of stacking fault becomes easier since sackingfault energy is lowered due to increase of Ti content. Dislocationmobility would be reduced by the dissociation of perfect dislocationinto partial dislocations and stacking fault surrounded by them. Lowdislocation mobility in γ′ enhances the resistance to creep deformation.Therefore, it is found that creep property is improved by an increase ofTi content.

As the present invention may be embodied in several forms withoutdeparting from the characteristics thereof, it should also be understoodthat the above-described embodiments are not limited by any of thedetails of the foregoing description, therefore, various variations arepossible by a person of ordinary skill in the pertinent art within therange of technical features of the present invention.

1. Ni-base single crystal superalloy with good creep property consistingof Co: 11.5˜13.5%, Cr: 3.0˜5.0%, Mo: 0.7˜2.0%, W: 8.5˜10.5%, Al:3.5˜5.5%, Ti: 2.5˜4.5%, Ta: 6.0˜8.0%, Re: 2.0˜4.0%, Ru: 0.1˜2.0% inWeight %, and the rest containing Ni and other inevitable impurities,and composition ratio of Al/Ti is 0.7˜2.2.
 2. The Ni-base single crystalsuperalloy with good creep property according to claim 1, wherein thesuperalloy has a mixed structure of the γ matrix and γ′ particles.